Insulated-gate field effect triode with an insulator having the same atomic spacing as the channel



United States Patent Ofiice 3,355,637 Patented Nov. 28, 1967 3,355,637 INSULATED-GATE WELD EFFECT TRIODE WITH AN INSULATQR HAVlNG THE SAME ATOMHC SPAUNG AS THE CHANNEL Harwiclr Johnson, Princeton, NJ, assignor to Radio Corporation of America, a corporation of Delaware Filed Apr. 15, 1965, Ser. No. 448,506 Claims. (Cl. 317-435) ABSTRACT 0F THE DISCLOSURE A more stable insulated gate field effect transistor has a gate insulator made of a material which has an atomic spacing approximately the same as that of the channel material, a conductance less than times the conductance of the channel material, and a bandgap, of at least 1.1 electron volts, which is at least 0.6 electron volts greater than the bandgap of the channel material and the conduction band edge of which is at an energy at least 0.4 electron volts higher than that of the conduction band edge in the channel material.

This invention relates to a novel field effect triode or transistor which has an insulated gate. Such device may be used as an active element in electronic circuits.

atoms which constitute the body material. As a result of the mismatch in atomic spacing, electronic states exist at the interface between the body and the insulator, which states adversely aifect the operation of the device.

An object of this invention is to provide a novel and improved insulated-gate field effect triode.

In general, the novel insulated-gate field eliect triode comprises a single crystal body of bandgap material having a surface, a source and a drain in spaced positions along the surface and defining the ends of a current path therebetween and a gate spaced from the current path by an insulator as do the previous devices. In the novel device, the insulator is of a crystalline bandgap material in which the average spacing of the atoms constituting the insulator closely matches that of the body material. Preferably, the average atomic spacing of the insulator material is between 0.9 and 1.1 times the atomic spacing of the body material. In addition, the conductance of the insulator material is preferably less than 10" times the conductance of the channel material, the bandgap of the insulator material is preferably at least 1.1 electron volts, and is at least 0.6 electron volt greater than that of the body material.

In the novel devices, the insulator material is preferably in the form of a single crystal grown into the same crystal with the body material. Such single crystal structure may be achieved by the epitaxial growth of the insulator upon a single crystal channel. Such structure provides improved production yields and more stable operating characteristics.

The novel devices exhibit substantially improved operating characteristics. For example, l/f noise is reduced and the operating characteristics of the triode remain more constant with time. The above-described structure may also be used to take advantage of the high carrier mobility existing in certain bandgap materials, for example, gallium arsenide.

A more detailed description of the improved device and illustrative embodiments thereof appear below in conjunction with the drawings in which:

FIGURE 1 is a sectional view of a first embodiment of the improved device together with a schematic circuit for operating the embodiment,

FIGURE 2 is a perspective View of an idealized structure presented for the purpose of defining various parameters used in an analysis of a triode,

FIGURE 3 is a portion of an idealized energy band diagram illustrating the relative positions of the bottoms of the conduction band edges of the insulator and the channel in a novel triode.

FIGURE 4 is an energy band diagram illustrating the relative energy levels in a typical section through the gate electrode, insulator, and channel of a triode embodiment of the improved device, and

FIGURE 5 is a family of illustrations, (A) through (E), illustrating the steps which may be used for preparing a triode such as the triode illustrated in FIGURE 1.

The invention is described for depletion-type devices having an N type channel. However, enhancement-type devices and devices having a P type channel are also part of the invention. Generally, the same analysis for circuits applies to devices having a P type channel except that the polarities of all applied voltages are reversed.

FIGURE 1 illustrates a device 21 which comprises a high resistivity single crystal body 23 of bandgap material, having a surface 22 and a source 25 and a drain 27 of conducting N type bandgap material in spaced locations along the surface 22. A layer of a crystalline insulator 29 (either single crystal or polycrystalline) of another bandgap material overlies the region 31 of the body 23 between the source 25 and the drain 27, which region is referred to as the channel 31. At gate 33, either of metal or of a conducting bandgap material (for example, a highly doped surface portion of the insulator 29), rests on the insulator 29 which spaces the gate 33 from the channel 31. The gate 33 may extend opposite part or all of the channel 31. Similarly, the insulator 29 may extend over part or all of the channel 31. A source contact 35 is in ohmic contact with the source 25, and a drain contact 37 is in ohmic contact with the drain 27.

The body 23 and the insulator 29 are of different bandgap materials which are related to one another by atomic spacing, conductance, and energy bandgap. The term bandgap material refers to any material having an energy bandgap between the valence and conduction bands in the energy levels in the material. Thus, the term includes what may be referred to as insulators and semiconductors, whose characteristics may or may not. be modified by the presence of impurities in the material.

To understand the relationship of atomic spacing, consider the following. If a mismatch in spacing of the atoms constituting the materials of the insulator 29 and the body 23 introduces surface states in the region of the interface therebetween to the extent of one state for each extra atom present, then the atomic spacing mismatch is limited to about 10% by the requirement that the surface state density for a good field effect device should be less than 10 cm?. Thus, the atomic spacing of the insulator material should be between 0.9 and 1.1 times the atomic spacing of the body material. A greater mismatch introduces too high a density of surface states, thereby degrading the operation of the device. The atomic spacings (lattice spacing) may be found in the literature for example, R. W. G. Wyckoft, Crystal Structures, John Wiley & Sons, New York, 1963.

To understand the relationships in conductance and bandgap, consider the following. The insulator 29 functions as a dielectric blocking layer so that the carrier currents through the insulator 29 between the gate 33 and the channel 31 are negligible compared with the currents through the channel. Tunnel currents between the gate 33 and the channel 31 are negligible by virtue of the thickness of the insulator layer (about 1000 Angstrom units or more). The intrinsic conductance of the insulator is small and injected currents are also small. For the first, compare ohmic conductance through the insulator 29 with the lateral conductance in a field effect channel. Referring to FIGURE 2, the ohmic current 1', (per unit channel Width) (1) Vd V2 where a field of Kiri 225 is taken as the effective field (Vg The lateral current i at saturation and V =0 is according to the present MOS theory. Comparing these and requiring that j,/j 10 we find that n lO cm.- under the following assumptions:

6:10, lL:2 10 cm., t=l0 cm., V V =l0 volts and a =l0 /=cm. =channel surface charge density. This requirement would be met by any pure material with a band-gap greater than 1.1 e.v. (room temperature operation).

To obtain an estimate of the barrier required to avoid injection currents, consider the conduction band barrier. Translating the surface density of cm. into a volume density of l0 /cm. We find a value of 0.4 e.v. if the density on the insulator side of the barrier is to be less than 10 cm. which we used previously, as shown in FIG- URE 3. Since the hole density will be so much less, the barrier for holes can be appropriately less. Again the barrier requirements depend on the temperature and are reduced exponentially for low temperature operation.

Referring to FIGURE 4, we have the following approximate picture of the minimum insulator requirements for room temperature operation.

The bandgap of the insulator (Eg is substantially equal to the sum of the bandgap of the semiconductor (Eg the difference in energy of the valence band edges in each of these materials (about 0.2 electron volt), and the difference in energy of the conduction band edges in each material (0.4 electron volt minimum). Thus, Eg, is about 0.6 e.v. greater than the bandgap of the semiconductor, and at least 1.1 e.v., as noted above. The gate 33 can be of a metal, or a surface portion of the insulator 29 that is very highly doped so the Fermi level is near the conduction band edge.

It is noteworthy that the dielectric constant of the insulator 29 is larger than that used in previous field effect triodes so the insulator 29 need not be as thin as used heretofore. However, conventional insulator thicknesses have been used in the calculations.

In the novel device structure, the channel 31 has a single crystal structure and the insulator 29 may have a single crystal structure also. The novel device is more stable and can be produced with improved production yields. The device may use III-V compounds and Group IV elements known to grow epitaxially, for example, as described in US. Patent 3,146,137 to F. V. Williams. For instance, one may use a GaP insulator on a GaAs channel,

a GaP insulator on a Si channel, a GaAs insulator on a Ge channel, or a GaAs insulator on an InAs or InSb channel and obtain the advantages of the high mobility of the channel materials.

The embodiment 21 of FIGURE 1 may be operated with a circuit 39 which comprises a source lead 41 conmeeting the source electrode 35 to ground 43, a gate section comprising a gate lead 45 connecting the gate electrode 33 to ground 43 through a gate bias source 49 and a signal source 47 connected in series, and a drain section comprising a drain lead 51 connecting the drain electrode 37 to ground 43 through a drain bias source 53 and a load resistor 55 connected in series. The output signal of the device may be taken across the load resistor 55 at terminals 57 on each side of the load resistor 55. An amplified replica of a signal applied to the gate 33 from the signal source 47 appears across the terminals 57. The polarity of the biases shown in FIGURE 1 are for operat' ing depletion type device 21 having an N type channel.

As shown in FIGURE 1, the body 23 is fioating (not connected to the circuit). Although not shown, the body 23 may also be biased, either with a DC or with an AC signal to provide an auxiliary input into the device. Also, if the body is thin and relatively resistive, an auxiliary gate electrode (not shown) may be positioned adjacent the body 23 opposite the gate 33 to provide an auxiliary signal input through an auxiliary gate electrode.

The devices of the invention include structures having channels 31 constituted of a single crystal, such as silicon, germanium, gallium arsenide, produced directly in a single crystal body or produced epitaxially on a single crystal support. For such single crystal structures, the insulator 29 may be deposited epitaxially from a vapor phase or in some materials, may be grown in situ. The insulator 29 may be polycrystalline or preferably single crystal. As explained above, the spacing of atoms in the insulator 29 is matched with the spacing of atoms in the channel material. The insulator 29 is preferably grown directly on the channel 31. In some embodiments, the channel material may be deposited upon the insulator, as compared with the insulator being deposited upon the channel as described above. The fabrication techniques for the insulated gate device of the invention are similar to those used to produce bipolar transistors and other electronic solid state devices. Impurity diffusion techniques may be used and the geometry may be controlled by precision masking and photolithographic techniques.

Referring now to FIGURE 5, a fabrication schedule for a device comprising :1 Gal insulator on a GaAs channel may be as follows. Start with a single crystal body 23 of P type gallium arsenide having a bulk resistivity between 10 and ohm-centimeters as shown in FIGURE 5A. Next, diffuse impurities into spaced regions in the body 23 to form the source 25 and the drain 27 by known N type diffusion and masking technique to provide the structure shown in FIGURE 53. For example, elemental sulfur may be diffused into gallium arsenide by heating a masked crystal in sulfur vapor for about 15 hours at about 1000 C. to produce the source and the drain. Next, an N type channel 31 is produced by diffusing impurities into the body 23 in the surface region between the source 25 and the drain 27. For example, by suitably masking the crystal and again heating the masked crystal in sulfur vapor for about one hour. The preferred heating times, temperatures, and vapor concentration for the sulfur diffusions should be determined empirically.

Then, grow epitaxially an insulator 29 of pure gallium phosphide by halogen transport or by oxide transport. For example, using a halogen transport technique, a thin gallium phosphide film can be formed by transport from a pure gallium phosphide source to the surface of the body 29 at a temperature of about 700 C. in an atmosphere of chlorine in several minutes. The structure at the end of this step is shown in FIGURE 5D and includes a gallium phosphide layer 29 about 1000 angstroms thick.

Then, form a gate electrode 33 on the insulator 29. For example, grow a highly doped (degenerate) layer 33 of gallium phosphide, as by using either a source highly doped with tellurium, or by adding an impurity such as sulphur to the hydrogen ambient. The gate electrode 33 and the insulator 29 are masked and etched to expose the source 25 and the drain 27. Then, source and drain contacts 35 and 37 are made either by direct connection of a metal lead or by evaporation of gold metal on the source and drain regions, respectively. A gate metallization 38 may be done during the same step. The completed structure is shown in FIGURE 5B.

Thus, techniques can be adapted from known procedures of diffusion, epitaxial growth, masking, etc., to fabricate the novel devices. These techniques can be used to fabricate otherexamples in this specification.

What is claimed is:

1. An insulated-gate field effect device comprising a single crystal body of bandgap material having a surface, a source and a drain in spaced positions along said surface and defining the ends of a current path in said body, and a gate spaced from said current path by an insulator, said insulator having a conductance less than times that of said body material, an atomic spacing between 0.9 and 1.1 times that of said body material, and a bandgap of at least 1.1 electron volts and at least 0.6 electron volt greater than that of said body material.

2. An insulated-gate field effect device comprising a single crystal body of bandgap material having a surface, a source and a drain in spaced positions along said surface and defining the ends of a current path in said body, and a gate spaced from said current path by an insulator, said insulator being in the form of a single crystal having a conductance less than 10 times that of said channel material, an atomic spacing between 0.9 and 1.1 times that of said body material and a bandgap of at least 1.1 electron volts and at least 0.6 electron volt greater than that of said body material.

3. An insulated-gate field effect device comprising a single crystal body of bandgap material having a surface, a source and a drain in spaced positions along said surface and defining the ends of a current path in said body and a gate spaced from said current path by an insulator, said insulator being a III-V compound in the form of a single crystal in the same crystal as said body material, said insulator having a conductance less than 10 times that of said body material, an atomic spacing between 0.9 and 1.1 times that of said body material and a bandgap of at least 1.1 electron volts and at least 0.6 electron volt greater than that of said body material.

4. An insulated-gate field effect device comprising a single crystal body of bandgap material having a surface, a source and a drain in spaced positions along said surface and defining the ends of a current path in said body, and a gate spaced from said current path by an insulator, said insulator having an atomic spacing between 0.9 and 1.1 times that of said body material.

5. An insulated gate field effect device comprising a single crystal body of bandgap material having a surface, a source and a drain in spaced positions along said surface and defining the ends of a current path in said body, and a gate spaced from said current path by an insulator, said insulator having a conductance less than 10* times that of said body material, an atomic spacing between 0.9 and 1.1 times that of said body material, and a bandgap of at least 1.1 electron volts, said bandgap having its conduction band edge at an energy level at least 0.4 electron volt higher than the conduction band edge of the bandgap of said body material.

References Cited UNITED STATES PATENTS 3,263,095 6/1966 Fang 307-885 JOHN W. HUCKERT, Primary Examiner.

M. EDLOW, Assistant Examiner. 

1. AN INSULATED-GATE FIELD EFFECT DEVICE COMPRISNG A SINGLE CRYSTAL BODY OF BANDGAP MATERIAL HAVING A SURFACE, A SOURCE AND A DRAIN IN SPACED POSITIONS ALONG SAID SURFACES AND DEFINING THE ENDS OF A CURRENT PATH IN SAID BODY, AND A GATE SPACED FROM SAID CURRENT PATH BY AN INSULATOR, SAID INSULATOR HAVING CONDUCTANCE LESS THAN 10-3 TIMES THAT OF SAID BODY MATERIAL, AN ATOMIC SPACING BETWEEN 0.9 AND 1.1 TIMES THAT OF SAID BODY MATERIAL, AND A BANDGAP OF AT LEAST 1.1 ELECTRON VOLTS AND AT LEAST 0.6 ELECTRON VOLT GREATER THAN THAT OF SAID BODY MATERIAL. 